Hi and welcome to Pearl-Guide!
Can you take a few photos of them against a white background (paper towel works fine), without flash?
Get some close up shots, in focus (use the little flower icon on your camera) especially any pearls with surface flaws (even minor ones). If there truly are no flaws, they were not knotted between the pearls (sounds like they weren't) and they are all 8mm, they are likely to be imitation.
Look carefully at the drill holes.
• Are they beveled at all? This would indicate imitation.
• Little swirls of excess coating near the drill hole can occur with imitation pearls.
Many imitations do not have either of these traits, however.
• Genuine pearls have very small drill holes; those of imitations tend to be larger.
Post a photo of all of them together, too, so we can look for overtones that might indicate they may be genuine cultured pearls. When the pearls are all too similar, it tends to indicate imitations.
If there is any peeling nacre anywhere, especially near the drill holes, photograph that, too. It will help us figure out what you have.
You may also wish to look at the photos on this thread about imitations-- some of them can be surprisingly real-looking:
https://www.pearl-guide.com/forum/showthread.php?t=7110
As far as grittiness, built-up grime can cause that sensation, or exerting too much pressure when rubbing.
As far as a gold clasp, even imitation jewelry was sometimes set in gold, when it was much less expensive. I have 18K gold jewelry from my grandmother set with imitation stones. However as you no longer have the clasp, is it possible it was just gold plated or gold filled?
But please, post some photos. It's all theoretical until then.
